1. Field of the Invention
The present invention relates to a trouble detection method and trouble detection apparatus for detecting a trouble in a control loop such as a disconnection or short circuit and a temperature controller, particularly to a trouble detection method and trouble detection apparatus preferred to detect a trouble such as a disconnection or short circuit in a control loop and a temperature controller.
2. Description of the Related Art
For example, as shown in FIG. 17, a temperature control system generally detects the temperature of a controlled system 1 such as a heating furnace by a temperature sensor 2 and inputs the temperature to a temperature controller 3′. The temperature controller 3′ performs a PID operation or the like in accordance with a set temperature (target value) and a detected temperature, outputs an operation signal to an SSR 4, and controls the power distribution to a heater 5 by an AC power supply 6 so that the temperature of the controlled system 1 becomes the set temperature.
The temperature controller 3′ has a sensor input circuit 7 to which an input is supplied from the temperature sensor 2, an A/D conversion circuit 8, a filter 9 according to software processing, control section 10 for performing an PID operation or the like, D/A conversion circuit 11, and output circuit 12.
In the case of the above temperature control system, to detect a trouble in a control loop such as disconnection of the heater 5 or temperature sensor 2 or short circuit of the SSR 4, it is necessary to detect a current by setting a current sensor to a place for detecting the current. Thus, the configuration becomes complex and the cost increases.
Therefore, an apparatus for detecting a trouble such as disconnection of a heater without using a current sensor is disclosed (for example, refer to Patent Document 1).
FIG. 18 is a block diagram showing a control loop system in the Patent Document 1.
A measured value PV from a controlled system 53 is input to a deviation circuit 57 and a trouble detection circuit 55 while an operation circuit 59 performs an PID operation in accordance with a deviation between a set value SV and the measured value PV to supply a manipulated variable MV to the controlled system 53 and the trouble detection circuit 55.
The trouble detection circuit 55 detects a trouble in the control loop system in accordance with the input manipulated variable MV and measured value PV.
That is, the trouble detection apparatus for the control loop system has counter means for counting predetermined trouble detection measurement time at a constant cycle, operation means for operating a manipulated variable for a controlled system in accordance with a measured value from the controlled system and a predetermined set value and outputting the operated manipulated variable to the controlled system, first determination means for determining whether the manipulated variable from the operation means is 0% or less or 100% or more, second determination means for determining a trouble depending on whether the measured value from the controlled system exceeds the range of a predetermined reference value for a measured value before start of the counting when the determination result by the first determination means is 0% or more or 100% or less of the manipulated variable and the counting means counts up the time, and trouble designation means for outputting a designation signal for designating a trouble in the control loop system of the controlled system when the measured value does not exceed the range of the predetermined reference value of the measured value before start of the counting in the second determination means.
The above configuration detects a trouble by counting a predetermined time from the point of time when a manipulated variable becomes 0% or less or 100% or more, detecting a change value of measured values at every predetermined time, and determining whether the change value exceeds a predetermined reference temperature range.
For example, in the normal state in which heating is continued by a heater up to a set temperature at a manipulated variable of 100%, a measured value rises over a predetermined reference temperature range every predetermined time. However, when the heater causes a disconnection, because the measured value cannot rise over the predetermined reference temperature range, it is possible to detect the trouble of heater disconnection.
Patent Document 1
U.S. Pat. No. 2,552,177
However, the above conventional example cannot detect a trouble unless a state does continues in which a manipulated value is 0% or less or 100% or more. Therefore, in a state in which the manipulated value is kept in a range of less than 100% over 0%, it is impossible to detect a trouble and thus trouble detection is delayed for a purpose requesting high-accuracy control.
Moreover, in the case of the above conventional example, when a set temperature corresponds to a manipulated variable of 100%, that is, when a manipulated variable in a steady state is 100%, for example, in a state in which a set temperature is set to 300° C. at a manipulated variable of 100%, the manipulated variable is 100% but temperature does not rise over a predetermined reference temperature range. Therefore, there is a difficult point that a trouble is erroneously determined.